Assymetric Paperboard

Abstract

There is provided a multi-layered paperboard comprising a first outer layer, a second outer layer and at least one middle layer arranged between the first and the second outer layer, wherein the grammage of the first outer layer is at least 2.0 times higher than the grammage of the second outer layer. A method of producing the multi-layered paperboard is also provided.

1 . A multi-layered paperboard comprising a first outer layer, a second outer layer and at least one middle layer arranged between the first and the second outer layer, wherein all these layers are based on cellulose fibers and wherein the grammage of the first outer layer is at least 2.0 times higher than the grammage of the second outer layer. 
     
     
         2 . The multi-layered paperboard of  claim 1 , wherein the grammage of the first outer layer is at least 2.5 times higher than the grammage of the second outer layer. 
     
     
         3 . The multi-layered paperboard of  claim 1 , wherein the density of the at least one middle layer is below 550 kg/m 3 . 
     
     
         4 . The multi-layered paperboard of  claim 1 , wherein the grammage of the multi-layered paperboard is 150-250 g/m 2  and the grammage of the middle layer is at least 2.5 times higher than the grammage of the second outer layer. 
     
     
         5 . The multi-layered paperboard of  claim 1 , wherein the grammage of the multi-layered paperboard is above 250 g/m 2  and the grammage of the middle layer is at least 4.0 times higher than the grammage of the second outer layer. 
     
     
         6 . The multi-layered paperboard of  claim 1 , wherein the grammage of the second outer layer is below 30 g/m 2 . 
     
     
         7 . The multi-layered paperboard of  claim 1 , wherein the grammage of the first outer layer is above 35 g/m 2 . 
     
     
         8 . The multi-layered paperboard of  claim 1 , wherein said least one middle layer is formed from at least one furnish comprising mechanical pulp. 
     
     
         9 . The multi-layered paperboard of  claim 7 , wherein said at least one furnish from which said at least one middle layer is formed further comprises broke pulp. 
     
     
         10 . A method of forming, in a full-scale paperboard machine, a multi-layered paperboard comprising a first outer layer, a second outer layer and at least one middle layer arranged between the first and the second outer layer, said method comprising the steps of:
 forming the first outer layer from a first furnish;   forming the second outer layer from a second furnish; and   forming said at least one middle layer from at least one middle layer furnish,   wherein the flows and consistencies of the first and the second furnish are such that the grammage of the first outer layer is at least 2.0 times higher than the grammage of the second outer layer.   
     
     
         11 . The method of  claim 10 , wherein the density of the at least one middle layer is below 550 kg/m 3 . 
     
     
         12 . The method of  claim 10 , wherein said at least one middle layer furnish comprises mechanical pulp. 
     
     
         13 . The method of  claim 10 , wherein the Schopper-Riegler number according to ISO 5267-1:1999 of the second furnish is above 25. 
     
     
         14 . The method of  claim 10 , wherein the Schopper-Riegler number according to ISO 5267-1:1999 is higher for the second furnish than for the first furnish. 
     
     
         15 . The method of  claim 10 , wherein the jet-wire speed difference in the formation of the first outer layer and in the formation(s) of the at least one middle layer is +/−5-30 m/min. 
     
     
         16 . The multi-layered paperboard of  claim 8 , wherein the mechanical pulp is chemitermomechanical pulp (CTMP). 
     
     
         17 . The method of  claim 12 , wherein the mechanical pulp is chosen from chemithermomechanical pulp (CTMP), or broke pulp.